Y. Kawamura’s research while affiliated with Kumamoto University and other places

We have identified a novel long-period stacking/order (LPSO) phase in a Mg-Ni-Y alloy, which provides novel LPSO structural features regarding both the stacking/order aspects; the 12R-type stacking sequence and the in-plane modulation of approximately 7×(12¯10) hcp with respect to the fundamental hexagonal-closed-packed Mg structure. The ideal 7 M model for the present 12R-type LPSO phase is constructed based on the ordered arrangements of the L1 2 -type Ni 6 Y 8 clusters embedded in the local ABCA stacking layers, resulting in the stoichiometry Mg 77 Ni 9 Y 12 (Mg 78.6 Ni 9.2 Y 12.2 ) that appears to be extremely close to the experimentally determined composition Mg 78.4 Ni 9.0±0.2 Y 12.6±0.3 (at.%). This in turn suggests the robustness of the solute cluster with a specific ratio (Ni 6 Y 8 ), emerging the pseudo-binary formation behaviors of the LPSO phase in the Mg ternary alloys.

The magnetic structure and spin-wave excitations in the quasi-square-lattice layered perovskite compound La$_5$Mo$_4$O$_{16}$ were studied by a combination of neutron diffraction and inelastic neutron scattering techniques using polycrystalline sample. Neutron powder diffraction refinement revealed that the magnetic structure is ferrimagnetic in the ab plane with antiferromagnetic stacking along the c axis where the magnetic propagation vector is $\mathbf{k}=\left(0,0,\frac{1}{2}\right)$. The ordered magnetic moments are estimated to be $0.54(2)\mu_\text{B}$ for Mo$^{5+}$ ($4d^1$) ions and $1.07(3)\mu_\text{B}$ for Mo$^{4+}$ ($4d^2$) ions at 4 K, which are about half of the expected values. The inelastic neutron scattering results display strong easy-axis magnetic anisotropy along the c axis due to the spin-orbit interaction in Mo ions evidenced by the spin gap at the magnetic zone center. The model Hamiltonian consisting of in-plane anisotropic exchange interactions, the interlayer exchange interaction, and easy-axis single-ion anisotropy can explain our inelastic neutron scattering data well. Strong Ising-like anisotropy and weak interlayer coupling compared with the intralayer exchange interaction can explain both the high-temperature magnetoresistance and long-time magnetization decay recently observed in La$_5$Mo$_4$O$_{16}$.

Inelastic X-ray scattering (IXS) experiments were performed on polycrystalline Mg97 Zn1 Y2 and Mg85 Zn6 Y9 alloys with synchronized long-period stacking ordered (LPSO) phase of about 24% and 100%, respectively, together with the reference pure Mg at room temperature for investigating the impurity effects in the microscopic elastic properties of the LPSO alloys. Inelastic neutron scattering (INS) measurements were also carried out on a polycrystalline Mg97 Zn1 Y2 alloy. The IXS measurements were carried out for the momentum transfers Q between 1.6 and 15.9 nm⁻¹ including the whole first- and partially second Brillouin zones, and for the energy transfers ω below 40 meV. Peaks arising from longitudinal acoustic (LA) modes were clearly observed in the IXS spectra of both the LPSO alloy and pure Mg, while transverse acoustic (TA) modes can mainly be detected in the second Brillouin zone. The dispersion relations in pure polycrystalline Mg are in good agreement with those of previous INS data on single crystal Mg. Broader inelastic signals and larger quasielastic peaks are characteristic for the LPSO alloys due to the phonon scattering by the Zn/Y impurities. Only the TA excitation energies increase by adding the impurities, which indicate a harder stiffness of the bond angles relating to the existence of the L12-type clusters in the LPSO alloys. Besides, new impurity-deriven dispersion-less excitation modes are observed at about 10 meV by adding the Zn/Y impurities. By comparing with the INS data on the polycrystalline LPSO alloy, the contributions of the impurities to these excitation modes are discussed using the differences in the scattering cross-sections between neutrons and X-rays.

div class="title">Investigation of Mg-Li-Ca alloys using a Wavelength Dispersive Soft X-ray Emission Spectrometer and EPMA - Volume 21 Issue S3 - H. Takahashi, M. Takakura, T. Murano, M. Terauchi, M. Yamasaki, Y. Kawamura, P. McSwiggen

The elastic properties of an Mg85Zn6Y9 (at.%) alloy single crystal with a long-period stacking-ordered (LPSO) structure, synchronized with periodic enrichment of Zn and Y atoms, were investigated, the properties having remained unclear because of the difficulty in growing large single crystals. Directionally solidified (DS) Mg85Zn6Y9 alloy polycrystals consisting of a single phase of the 18R-type LPSO structure were prepared using the Bridgman technique. For the DS polycrystals, a complete set of elastic constants was measured with resonant ultrasound spectroscopy combined with electromagnetic acoustic resonance, in which the texture formed by the directional solidification was taken into account. By analyzing the elastic stiffness of DS polycrystals on the basis of a newly developed inverse Voigt Reuss Hill approximation, the elastic stiffness components of the single-crystalline LPSO phase were determined. It was revealed that the Young's modulus of the LPSO phase along (00 01) in the hexagonal expression was clearly higher than that along < 1 1 (2) over bar0 >, and the Young's modulus and shear modulus were clearly higher than those of pure magnesium. These findings were validated by first-principles calculations based on density functional theory. Analyses by first-principles calculations and micromechanics modeling indicated that the long periodicity of the 18R-type stacking structure hardly enhanced the elastic modulus, whereas the Zn/Y-enriched atomic layers, containing stable short-range ordered clusters, exhibited a high elastic modulus, which contributed to the enhancement of the elastic modulus of the LPSO phase in the Mg-Zn-Y alloy.

Mg97Zn1Y1RE1 (RE=La, Ce, Nd and Sm, at. %) alloys were prepared by high-frequency induction melting in an Ar atmosphere. Rods were extruded at 623 K and a ram speed of 2.5 mm·s−1 using a circular die with an extrusion ratio of 10. The microstructure and mechanical properties of the extruded alloys were investigated. The Mg97Zn1Y1Nd1 and Mg97Zn1Y1Sm1 alloys consisted of only two phases: α-Mg and a Mg-RE intermetallic compound. The Mg97Zn1Y1La1 and Mg97Zn1Y1Ce1 alloys consisted of three phases: α-Mg, a Mg-RE intermetallic compound, and a Mg12ZnY phase with a long-period stacking ordered (LPSO) phase. Additionally, after extrusion, the three-phase Mg97Zn1Y1RE1 alloys, i.e., those with an LPSO phase, had a stratified microstructure and exhibited better mechanical properties than those without an LPSO. At room temperature, the yield strength and ultimate tensile strength of the three-phase Mg97Zn1Y1La1 and Mg97Zn1Y1Ce1 alloys were 381–384 MPa and 427–429 MPa, respectively, and yield strengths greater than 280 MPa were observed at the elevated temperature of 523 K.

Non-basal slip systems in the Mg12ZnY long-period stacking ordered (LPSO) phase, the operational frequency of which is increased at high-temperatures and affects the mechanical properties, were clarified. The {1 (1) over bar 00} < 11 (2) over bar0 > prism slip was identified in both 18R and 14H LPSO phases, even though they have the different lattice systems. This behavior is different from that observed in a Ni-based LPSO phase. The peculiar chemical modulation in the Mg12ZnY LPSO phase may affect the selection of operative slip systems.

In this study, the microstructure and mechanical properties of MgZnY alloy sheets were investigated. Tensile tests at room temperature were performed along the rolling direction of Mg98Zn1Y1-, Mg96Zn2Y2-, and Mg94Zn3Y3- alloy sheets and their annealed states (773 K for 0.6 ks). These alloy sheets exhibited yield strengths of 261, 317, and 380 MPa, and elongations of 12, 10, and 6%, respectively. The yield strength of a MgZnY alloy sheet with Zn and Y contents greater than 2 at% was higher than 300 MPa. The microstructure observations suggested that the alloy sheet strength mainly resulted from (i) the formation of basal texture in the long period stacking ordered (LPSO) phase and (ii) the uniform dispersion of a fine Mg3Zn3Y2 phase. In the annealed state, the yield strength tended to decrease, while the elongation tended to increase. Large elongations of 20% or more were achieved in the Mg98Zn1Y1- and Mg96Zn2Y2 -alloy annealed sheets. The cold workability of the MgZnY alloy sheets and an AZ31-O sheet were evaluated, using a V-bending test at room temperature. Both Mg98Zn1Y1- and Mg96Zn2Y2- annealed sheets could be bent without cracking with a minimum bending radius per thickness of 3.3, which was less than that of the AZ31-O sheet. Texture randomization occurred in the MgZnY alloy annealed sheets owing to re-crystallization of the Mg phase, which was confirmed by electron backscattering diffraction (EBSD) analysis. Large elongations and good cold workability of the MgZnY annealed sheets are presumably attributed to an increase in the randomness of the Mg phase owing to re-crystallization. These results suggested that a Mg alloy sheet of high yield strength or good cold workability could be prepared by controlling the alloy composition and its microstructure in the MgZnY alloy system.

It has recently been found that Mg97Zn1Y2 extruded alloy containing a long-period stacking ordered (LPSO) phase has superior mechanical properties. In this study, the high-temperature deformation mechanism of the Mg97Zn1Y2 extruded alloy was examined. Grain-boundary strengthening due to the refined Mg-matrix phase and fiber-like reinforcement due to the LPSO phase dominantly contribute to the strengthening of the alloy at room temperature, and they were confirmed to effectively act even at 200 degrees C. As a result, the extremely high strength of the alloy is maintained up to 200 degrees C, unlike other conventional Mg alloys. At 300 degrees C, however, the yield stress of the Mg97Zn1Y2 alloy largely decreases, and the orientation and the grain size dependence of the yield stress become weak. Increases in the operation frequency of non-basal slip in the Mg-matrix grains weaken the grain-boundary strengthening effect. In addition, the effect of fiber-like reinforcement due to the LPSO phase is also weakened at 300 degrees C because the window of microstructure suitable for inducing this effect becomes significantly narrow at this temperature.